1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrographic copying machine, a printer and a facsimile machine, and more particularly, to an image forming apparatus in which the precision of transfer of toner images formed on the surface of an electrostatic latent image carrier is improved.
2. Description of the Prior Art
As an image forming apparatus of the above-mentioned type, an apparatus is widely known which is provided with a transfer section employing a contact-type bias roller transfer method using a conductive roller. In the transfer section according to this method, as schematically shown in FIG. 1, in a transfer area of a photoreceptor drum (electrostatic latent image carrier) 31 where a transfer sheet S on which toner images are to be transferred passes, a transfer roller 32 serving as a charge supplying means is arranged to be always in contact with the drum 31 while rotating in a direction corresponding to the direction of rotation of the drum 31, and a transfer voltage of a polarity reverse to that of the toner is applied to the axial core of the transfer roller 32. Reference numeral 33 represents a power source for the voltage application to the axial core of the roller 32.
In the conventional transfer section of the above-described arrangement, the transfer sheet S passing between the surfaces of the transfer roller 32 and the drum 31 is pressed against the toner adhering to the drum surface, and the toner image formed on the drum surface is transferred onto the transfer sheet S by applying the transfer voltage of a polarity reverse to that of the toner to the axial core of the roller 32.
In the transfer section employing the bias roller transfer method, however, since the surface of the drum 31 and the surface of the transfer roller 32 are pressed against each other with the transfer sheet S therebetween, it is apt to occur that the toner located in a central portion of the drum surface is not transferred onto the transfer sheet S and that toner scatters and adheres to peripheral portions of the image transferred onto the transfer sheet S.
To solve such problems of the bias roller transfer method, the present applicant proposed an arrangement as schematically shown in FIG. 2 in Japanese Patent Application H4-284120. In this art a transfer roller 42 is arranged so that a gap larger than the thickness of the transfer sheet S is left between a drum 41 and the roller 42 in order that the surface of the roller 42 is always out of contact with the drum surface, and the transfer sheet S brought in close contact with the drum surface is kept out of contact with the transfer roller 42 to prevent the above-mentioned problems.
In the case of this prior art, it is considered that the reason why the toner images on the surface of the drum 41 are satisfactorily and excellently transferred onto the transfer sheet S although the transfer roller 42 is out of contact with the surface of the drum 41 is that a slight corona discharge is generated from the transfer roller 42 to the reverse surface of the transfer sheet S since a transfer voltage of a polarity reverse to that of the toner is applied from a power source 43 to the transfer roller 42. Actually, excellent test results have been obtained with respect to the performance of toner image transfer.
However, since the transfer roller 42 is arranged to be out of contact with the drum surface and the transfer sheet S is not supported from the reverse side by the transfer roller 42, the movement of the transfer sheet S is unstable, and the contact pressure between the transfer sheet S and the surface of the drum 41 is not kept constant. As a result, there is unevenness in the transferred image.
In the above prior art, a resist roller pair 41 sends out the transfer sheet S at a speed equal to the peripheral speed of the drum. The transfer sheet S is guided along the upper surface of a lower guide member 45 toward the drum 41 while being prevented by an upper guide member 46 from being separated from the upper surface of the lower guide member 45.
The transfer sheet S adheres to the surface of the drum 41 due to electrostatic force after its front end passes the end of the lower guide member 45 and abuts the surface of the drum 41, and moves at a speed equal to the peripheral speed of the drum 41. While the transfer sheet S is in close contact with the surface of the drum 41, a transfer voltage is applied to the transfer roller 42 in order to transfer the toner image from the surface of the drum 41 to the transfer sheet S.
The transfer sheet S which has been separated from the surface of the drum 41 moves along the sheet conveying path towards the downstream side while being weighed down by its own dead load, although it depends on its length. The transfer sheet S is released from the resist roller pair 44 after it reaches the upper surface of a guide table 47, and is conveyed along the guide table 47 to a fixing roller pair (not shown) in accordance with the rotation of the drum 41. As described above, the transfer sheet S bends in a direction opposite to the curve of the drum surface between the lower guide member 45 and the guide table 47. The resiliency of the transfer sheet S generated by the bend works as the contact pressure on the portion of the transfer sheet S which is in contact with the surface of the drum 41.
The conveying speed of the transfer sheet S and the angle of inclination of the upper surface of the lower guide member 45 which guides the transfer sheet S toward the drum 41 are set so that conditions are fulfilled for making the transfer sheet S to be in close contact with the surface of the drum 41 while being conveyed at a speed equal to the peripheral speed of the drum surface. In actuality, however, the resiliency of the transfer sheet S varies according to environmental conditions such as the temperature and humidity around the transfer area.
For this reason, under an environmental condition where the above setting conditions are not fulfilled, the transfer sheet S may not be in contact with the drum surface at a predetermined contact pressure since the transfer sheet S is not sufficiently resilient. Moreover, since some accumulative errors are caused in the sheet feeding speed of feeding roller pairs provided at a plurality of positions in a paper feeding mechanism (not shown) arranged on the upstream side of the transfer section, it is unavoidable that the position of the transfer sheet S deviates relative to the position of the toner image on the surface of the drum 41.
Such error factors relating to paper feeding cause no problems in the case of the conventional contact-type transfer roller 32 shown in FIG. 1, since the roller 32 rotates synchronously with the drum 31 to restrict the conveying speed of the transfer sheet S. However, when the transfer roller 42 is out of contact with the drum 41 like in the prior art of FIG. 2, it is difficult to eliminate the generation of unevenness in the transferred image since no means is provided for forcibly regulating the conveying speed and the transfer timing of the transfer sheet S.
The sheet which has been separated from the drum 41 is pulled by the fixing roller pair. Since the peripheral speed slightly differs between the fixing roller pair and the resist roller pair 44, the transfer sheet S may excessively be pulled by the fixing roller pair or may be bent between the drum 41 and the fixing roller pair. However, regarding this, normally, the diameter of the rollers constituting the resist roller pair 44 and the fixing roller pair are each set so that the transfer sheet S is slightly bent between the roller pairs.
When the transfer sheet S is short, the rear end thereof may be separated from the resist roller pair 44 while transfer is still being performed. For such a case, a fact is utilized that the transfer sheet S is attracted to the drum surface by electrostatic force. The rollers and guide members are so arranged that the attractive force keeps the transfer sheet S free from position shift or disorder all the way to the fixing roller pair.
In the arrangement of the above prior art, the movement speed of the transfer sheet S and the peripheral speed of the drum surface are maintained coincident with each other while the resist rollers 44 is nipping the transfer sheet S as shown in FIG. 2. However, after the rear end of the transfer sheet S is released from the resist roller pair 44, the movement speed of the transfer sheet S slightly changes since the transfer sheet S comes into close contact with the upper guide member 46 due to its resiliency as shown in FIG. 3. As a result, in the rear end portion of the transfer sheet S, the latent image formed on the drum surface is transferred with its position slightly shifted.
Further, as shown in FIG. 4, after the rear end of the transfer sheet S leaves the end of the lower guide member 45, a downward force due to the dead load works on the rear end portion of the transfer sheet S. Therefore, the contact pressure between the transfer sheet S and the surface of the drum 41 is reduced largely, though the foregoing portion of the transfer sheet S still adheres to the surface of the drum 41 by the electrostatic force. Because of this, the toner image is not transferred correctly in the rear end portion of the transfer sheet S.
Thus, when a high definition image is required, it is difficult in the above prior art to realize a sufficient transfer precision only by setting the arrangement of the rollers and guide members accurately or varying the positions of the guide members.